Abstract It has been suggested that the line width of ions in molecular clouds is narrower than that of the coexisting neutral particles, which has been interpreted as an indication of the decoupling of neutral turbulence from magnetic fields within a partially ionized environment. We calculate the Principal Component Analysis (PCA) correlation coefficients of CCH vs. H13CO+ and H13CN vs. H13CO+. We find aside from H13CN, CCH could also be strongly spatial correlated with H13CO+ in high-mass star forming regions. CCH and H13CO+ line emission are strongly spatial correlated with each other in 48% sources with a PCA correlation coefficient over 0.7. So we investigate the ambipolar diffusion (AD) effect using CCH and H13CO+ lines as a neutral/ion pair in a sample of 129 high-mass star forming clumps. We conduct a careful analysis of line widths of the CCH-H13CO+ pair pixel-by-pixel in 12 sources, which show strong correlation in CCH-H13CO+ emission and no obvious outflows or multiple velocity components. The mean velocity dispersion of CCH is about the same as H13CO+ in 12 sources. In low-density regions of most sources, CCH shows a broader velocity dispersion than H13CO+. However, the AD effect is not significant from a statistical point of view.

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions – XVI. Neutral versus ion line widths